From: Zhenzhong Duan <zhenzhong.duan@oracle.com>
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
linux-kernel@vger.kernel.org
Cc: vkuznets@redhat.com, linux-hyperv@vger.kernel.org,
kvm@vger.kernel.org, kys@microsoft.com, haiyangz@microsoft.com,
sthemmin@microsoft.com, sashal@kernel.org, tglx@linutronix.de,
mingo@redhat.com, bp@alien8.de, pbonzini@redhat.com,
rkrcmar@redhat.com, sean.j.christopherson@intel.com,
wanpengli@tencent.com, jmattson@google.com, joro@8bytes.org,
jgross@suse.com, sstabellini@kernel.org, peterz@infradead.org,
Jonathan Corbet <corbet@lwn.net>,
"H. Peter Anvin" <hpa@zytor.com>, Will Deacon <will@kernel.org>
Subject: Re: [PATCH v4 1/4] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks
Date: Sun, 6 Oct 2019 15:49:01 +0800 [thread overview]
Message-ID: <b0d6df7b-00ff-cdd8-f9f2-26af73256f5b@oracle.com> (raw)
In-Reply-To: <26ef7beb-dad0-13c9-fc2f-217a5e046e4d@oracle.com>
On 2019/10/4 22:52, Boris Ostrovsky wrote:
> On 10/3/19 10:02 AM, Zhenzhong Duan wrote:
>> void __init kvm_spinlock_init(void)
>> {
>> - /* Does host kernel support KVM_FEATURE_PV_UNHALT? */
>> - if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
>> - return;
>> -
>> - if (kvm_para_has_hint(KVM_HINTS_REALTIME))
>> + /*
>> + * Don't use the pvqspinlock code if no KVM_FEATURE_PV_UNHALT feature
>> + * support, or there is REALTIME hints or only 1 vCPU.
>> + */
>> + if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT) ||
>> + kvm_para_has_hint(KVM_HINTS_REALTIME) ||
>> + num_possible_cpus() == 1) {
>> + pr_info("PV spinlocks disabled\n");
>> return;
>> + }
>>
>> - /* Don't use the pvqspinlock code if there is only 1 vCPU. */
>> - if (num_possible_cpus() == 1)
>> + if (nopvspin) {
>> + pr_info("PV spinlocks disabled forced by \"nopvspin\" parameter.\n");
>> + static_branch_disable(&virt_spin_lock_key);
> Would it make sense to bring here the other site where the key is
> disabled (in kvm_smp_prepare_cpus())?
Thanks for point out, I'll do it. Just not clear if I should do that in a separate patch,
there is a history about that code:
Its original place was here and then moved to kvm_smp_prepare_cpus() by below commit:
34226b6b ("KVM: X86: Fix setup the virt_spin_lock_key before static key get initialized")
which fixed jump_label_init() calling late issue.
Then 8990cac6 ("x86/jump_label: Initialize static branching early") move jump_label_init()
early, so commit 34226b6b could be reverted.
>
> (and, in fact, shouldn't all of the checks that result in early return
> above disable the key?)
I think we should enable he key for !kvm_para_has_feature(KVM_FEATURE_PV_UNHALT) case,
there is lock holder preemption issue as qspinlock is fair lock, virt_spin_lock()
is an optimization to that, imaging one pcpu running 10 vcpus of same guest
contending a same lock.
For kvm_para_has_hint(KVM_HINTS_REALTIME) case, hypervisor hints there is
no preemption and we should disable virt_spin_lock_key to use native qspinlock.
For the UP case, we don't care virt_spin_lock_key value.
For nopvspin case, we intentionally check native qspinlock code performance,
compare it with PV qspinlock, etc. So virt_spin_lock() optimization should be disabled.
Let me know if anything wrong with above understanding. Thanks
Zhenzhong
next prev parent reply other threads:[~2019-10-06 7:51 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-03 14:02 [PATCH v4 0/4] Add a unified parameter "nopvspin" Zhenzhong Duan
2019-10-03 14:02 ` [PATCH v4 1/4] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks Zhenzhong Duan
2019-10-04 14:52 ` Boris Ostrovsky
2019-10-06 7:49 ` Zhenzhong Duan [this message]
2019-10-07 14:46 ` Boris Ostrovsky
2019-10-08 2:17 ` Zhenzhong Duan
2019-10-03 14:02 ` [PATCH v4 2/4] x86/kvm: Change print code to use pr_*() format Zhenzhong Duan
2019-10-03 14:02 ` [PATCH v4 3/4] xen: Mark "xen_nopvspin" parameter obsolete Zhenzhong Duan
2019-10-04 14:57 ` Boris Ostrovsky
2019-10-06 7:52 ` Zhenzhong Duan
2019-10-03 14:02 ` [PATCH v4 4/4] x86/hyperv: Mark "hv_nopvspin" " Zhenzhong Duan
2019-10-05 17:07 ` Michael Kelley
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